It is known in connection with continuous casting processes that numerous defects arise in the cast billets, blooms or ingots, particularly in the form of segregation cracks, core segregations and inclusions of nonmetallic substances. Because of the differences in the cooling conditions during continuous casting and ordinary casting, such defects have been found to be particularly significant in continuous casting billets.
Attempts have been made to reduce the defects of continuous casting by optimally adjusting and maintaining the casting temperature, the casting speed and cooling intensity for the continuously cast strand. This has been found to be a difficult procedure.
The predominant defect in continuous casting is that which is brought about by segregations or oxidic inclusions. The segregating elements tend to be enriched in the melt above the solidifying casting and are eventually entrained in the form of nonmetallic inclusions as solidification proceeds. It has been found that segregation and oxidic inclusion defects are a function of the residual oxygen and sulfur contents of the melt. Enrichment of such elements tends to occur in the core of the continuous casting, resulting in an increase in the formation of oxides, sulfides, gas-bubble spaces and a globulitic solidification structure. These all are associated with core segregation.
The invention is based upon the principle that it is possible to avoid such defects by providing a casting melt which contains practically no solubilized oxygen and no sulfur.
It has been found that prior-art techniques for the reduction of the oxygen and sulfur levels in a steel melt are not satisfactory in reducing the concentrations of these elements to the desired low level.
For example, it has been proposed to produce a steel melt with an extremely low sulfur content using techniques which involve the addition of cerium mix metal (misch metal) having a high affinity for sulfur or by blowing calcium components into the steel melt.
It is also known to reduce the oxygen content of a steel melt by introducing compounds or elements with a high oxygen affinity into the melt. Such substances are, for example, silicon and aluminum.
Both techniques have been found to have certain disadvantages. For example, when silicon is introduced into the melt it is not possible to lower the oxygen level sufficiently, presumably because silicon does not have a sufficient affinity for oxygen. The use of a stronger deoxidizing medium such as aluminum has the disadvantage that a portion of the reaction product, namely aluminum oxide, does not separate from the melt but remains dispersed in the liquid phase and is entrained therewith into the casting. Electrochemical techniques have shown that oxygen does not react with aluminum completely when it is used as the deoxidizing agent. Thus, in order to reduce the oxygen level sufficiently, it is necessary to use 10 to 15 times as much aluminum as is theoretically necessary to combine with all of the oxygen. Very low residual oxygen contents, e.g. below 10 parts per million, can thus only be achieved with extremely high aluminum quantities or by the addition of still more effective deoxidizing agents such as cerium mix metal.
The latter technique, however, gives rise to a problem which has long been recognized and feared in the art, namely, the reaction of the excess deoxidizing agent (aluminum or cerium) during the casting with the refractory lining of the casting system and especially the silica thereof. This reaction produces additional oxides which eventually are incorporated in the steel melt and are found in the casting. Furthermore, the suspended oxide aggregates in the melt increase the viscosity thereof so that higher melt temperatures must be used to cast the steel.
In continuous casting, an increase of the casting temperature has significant disadvantages, e.g. greater wear of the casting system, less effective cooling and solidification, etc. Furthermore, the oxide aggregates can deposit on the wall of the casting system, can create blockages, and can be incorporated in the casting both along the periphery and within the interior thereof.
It has been found to be necessary, in such cases to machine the surfaces of the cast ingot or billet at considerable cost and with significant losses of material.
Efforts have also been made to remove the detrimental alumina particles from the steel band. For example, German published application (Offenlegungsschrift) No. 2,304,943 teaches the introduction of a lance or the like into the melt so as to induce the oxide which would be entrained into the continuous casting strand to rise to the casting slag covering the melt. Another proposal (German published application - Offenlegungsschrift - No. 2,300,963) induces the deposition of the alumina aggregates on a lattice-like arrangement of refractory ceramic material.
In German published application (Offenlegungsschrift) No. 2,312,137, the oxides suspended in the melt are induced to flow in a given manner and to separate from the steel. German published application (Offenlegungsschrift) No. 2,219,818 suggests that the problem can be eliminated by separating the oxides from the melt by the use of a jet of a purifying gas.
Finally, there may be mentioned a number of other techniques which have been proposed in order to avoid entrainment of the alumina aggregates into the melt, namely, the flushing of the melt with inert gas, the treatment of a melt in a vacuum or either of these techniques in combination with the purification or refining approaches mentioned above. All of these techniques have been found to have various disadvantages or involve prohibitive costs. Frequently they require increasing the casting temperature, etc.
The above-mentioned enumeration of techniques which have been used to solve this problem demonstrates that the art has not yet been able to produce a melt for continuous casting in an inexpensive, efficient and problem-free manner.